The liquid present in the intermediate chamber presents a volume that decreases for three main reasons: the existence of leaks, the existence of dissolved gas that affects the performance of the pump, and the presence of a safety valve that enables fluid to be discharged in the event of excess pressure.
With this type of pump, it is therefore necessary to provide a system for topping up the chamber in question, generally from an auxiliary tank. Said system, referred to as a compensation system, should maintain a volume of liquid in the intermediate chamber that makes it possible under all working conditions for the diaphragm to travel through a distance corresponding to the cylinder capacity swept by the piston, and without running the risk of damaging the diaphragm or disturbing the flow.
Known top-up devices are of two main types: automatic devices and controlled devices. Automatic devices are constituted by a simple rated suction valve that opens from the tank to the intermediate chamber as from a certain level of reduced pressure. Controlled devices comprise one or more valves that are opened mechanically by the movement of the diaphragm and that enable topping up to take place only when the diaphragm is in its extreme rear position.
In both cases, the diaphragm is deformable without significant stiffness or elasticity, such that the suction level of the pump is a function of the pressure in the intermediate chamber. Under such conditions, automatic systems suffer from the drawback of greatly decreasing the suction capacity of the pump compared with that of a piston pump, since the valve needs to be rated so that it opens as late as possible before cavitation occurs in the intermediate chamber, and suction ceases immediately at the moment that this opening occurs. If the diaphragm has not traveled over its full suction stroke, then the cylinder capacity of the pump is affected.
Certain known controlled systems include a valve that co-operates with a stationary seat through which the top-up duct opens out into the intermediate chamber, the valve being urged against its seat by a spring and being opened by the diaphragm when it tends to go past the end of the pump suction stroke. In most circumstances, the diaphragm is flexible with practically no stiffness.
In certain low-flowrate pumps, the diaphragm used is in is the form of a dome or a cone that is elastically deformable, presenting great stiffness and possessing a memory of its rest shape that corresponds to the end of the suction stroke and to which it returns elastically when pressure ceases to exist in the intermediate chamber.
The small cylinder capacity of such pumps prevents known devices for performing the top-up function from being transposed simply. It is therefore appropriate to adapt the compensation system to the particular conditions in which such pumps operate. The invention consists in making this adaptation and it leads to miniaturization of the circuit for compensating the working chamber, the invention also presenting advantages, in particular a reduction in size, in pumps that present higher flow rates.